Martin got GMCS to work on Windows, with a small caveat — there’s no API to change the MethodAttributes of a MethodBuilder. I have a couple of observations:

the caveat isn’t really a big deal. You only need to set NewSlot when there is a new keyword used in the declaration. Otherwise, it’s useful but not necessary (See bug 74852 for the gory details)

these changes, coupled with my older changes should allow us to have more code similarity between MCS and GMCS. Specifically, I think we can put in a couple of stubs: DropGenericTypeArguments and DropGenericMethodArguments into MCS and eliminate a big part of the textual differences between MCS and GMCS in non-generics related code. We already started on this in doc.cs